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August 22, 2006IAU Symposium 239 Observing Convection in Stellar Atmospheres John Landstreet London, Canada.

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Presentation on theme: "August 22, 2006IAU Symposium 239 Observing Convection in Stellar Atmospheres John Landstreet London, Canada."— Presentation transcript:

1 August 22, 2006IAU Symposium 239 Observing Convection in Stellar Atmospheres John Landstreet London, Canada

2 August 22, 2006IAU Symposium 239 Introduction Convection reaches photosphere in most stars of T e < 10 4 K, perhaps also in hotter stars Convection reaches photosphere in most stars of T e < 10 4 K, perhaps also in hotter stars Directly visible in Sun as granulation Directly visible in Sun as granulation Detected in stars as microturbulence, macroturbulence, bisector curvature, etc Detected in stars as microturbulence, macroturbulence, bisector curvature, etc Comparison of convection models with observed spectra provides interpretation of observations and tests of models Comparison of convection models with observed spectra provides interpretation of observations and tests of models

3 August 22, 2006IAU Symposium 239 Solar granulation Appearance of sun with good seeing reveals granulation Appearance of sun with good seeing reveals granulation Sequences of images suggest coherent overturning flow Sequences of images suggest coherent overturning flow Granulation ~ visible convection cells Granulation ~ visible convection cells => Study convection observationally => Study convection observationally

4 August 22, 2006IAU Symposium 239 Indirect detection of velocity fields Granulation not directly visible on (unresolved) stellar surfaces Granulation not directly visible on (unresolved) stellar surfaces But velocity fields in photosphere affect spectral line profiles & energy distribution, so we may still study convection observationally But velocity fields in photosphere affect spectral line profiles & energy distribution, so we may still study convection observationally Simplest example of velocity field: stellar rotation Simplest example of velocity field: stellar rotation Small for “cool” stars, large for “hot” stars Small for “cool” stars, large for “hot” stars

5 August 22, 2006IAU Symposium 239 Microturbulence Abundance analysis allows indirect detection of small-scale velocity field (excess line broadening over thermal), required to fit weak and strong lines Abundance analysis allows indirect detection of small-scale velocity field (excess line broadening over thermal), required to fit weak and strong lines Microturbulence parameter  characterizes velocity Microturbulence parameter  characterizes velocity Required for most stars with T e < K, corresponds to convective instability Required for most stars with T e < K, corresponds to convective instability => Microturbulence ~ convection, at least in cooler stars => Microturbulence ~ convection, at least in cooler stars Detectable even in broad-line stars – much data Detectable even in broad-line stars – much data

6 August 22, 2006IAU Symposium 239 Convection effects on line profiles In Sun-like flow, expect rising and descending gas to have different velocities along line of sight In Sun-like flow, expect rising and descending gas to have different velocities along line of sight Different areal coverage (filling factors) and brightness lead to different contributions to total flux Different areal coverage (filling factors) and brightness lead to different contributions to total flux Result: spectral lines are shifted and asymmetric Result: spectral lines are shifted and asymmetric Importance of these effects depends on where in atmosphere the lines are formed – weak lines will be different from strong lines Importance of these effects depends on where in atmosphere the lines are formed – weak lines will be different from strong lines

7 August 22, 2006IAU Symposium 239 Macroturbulence Most main sequence stellar line profiles can be roughly modelled with Voigt profile + rotation Most main sequence stellar line profiles can be roughly modelled with Voigt profile + rotation Line profiles of giants & supergiants more “pointed”, with broad shallow wings Line profiles of giants & supergiants more “pointed”, with broad shallow wings Successful model: radial-tangential macroturbulence. On half of surface, lines have Gaussian spread radially, on other half lines have Gaussian spread tangentially. Successful model: radial-tangential macroturbulence. On half of surface, lines have Gaussian spread radially, on other half lines have Gaussian spread tangentially. One parameter: macroturbulence  RT (velocity) One parameter: macroturbulence  RT (velocity)

8 August 22, 2006IAU Symposium 239 Macroturbulence - 2 If  RT > 0, we conclude that large-scale velocity field exists within stellar atmosphere If  RT > 0, we conclude that large-scale velocity field exists within stellar atmosphere => Velocity field may be studied by modelling spectral line shapes => Velocity field may be studied by modelling spectral line shapes Values vary systematically over cooler part of HR diagram Values vary systematically over cooler part of HR diagram Large values of macroturbulence found among (low v sin i) main sequence A stars near T e ~ 8000K Large values of macroturbulence found among (low v sin i) main sequence A stars near T e ~ 8000K Macroturbulence drops to zero above A0V Macroturbulence drops to zero above A0V

9 August 22, 2006IAU Symposium 239 Macroturbulence - 3 Among hotter stars (T e > 10000) situation is quite confusing Among hotter stars (T e > 10000) situation is quite confusing For hot main sequence and giant stars, microturbulence takes various values between 0 and several km/s, but not systematically – are these values really > 0 (e.g. Lyubimkov et al 2004)? For hot main sequence and giant stars, microturbulence takes various values between 0 and several km/s, but not systematically – are these values really > 0 (e.g. Lyubimkov et al 2004)? B and A supergiants have microturbulence of several km/s, and macroturbulence of 15 – 20 km/s (e.g. Przybilla et al 2006)! B and A supergiants have microturbulence of several km/s, and macroturbulence of 15 – 20 km/s (e.g. Przybilla et al 2006)! Is supergiant macroturbulence due to winds, non-radial pulsations, convection, or…? Is supergiant macroturbulence due to winds, non-radial pulsations, convection, or…?

10 August 22, 2006IAU Symposium 239 Radial velocities In convecting stars (  > 0), radial velocity of lines observed to vary with line strength In convecting stars (  > 0), radial velocity of lines observed to vary with line strength Reflects typical velocity (average over flows) at depth where line is formed Reflects typical velocity (average over flows) at depth where line is formed Difficult to study: requires very accurate lab wavelengths, sharp lines Difficult to study: requires very accurate lab wavelengths, sharp lines Not yet studied over full HR diagram Not yet studied over full HR diagram Examples: Sun, Procyon (Allende Prieto et al 2002) Examples: Sun, Procyon (Allende Prieto et al 2002)

11 August 22, 2006IAU Symposium 239 Bisector curvature (asymmetry) Line asymmetry (bisector curvature) reveals asymmetric flows Line asymmetry (bisector curvature) reveals asymmetric flows Should provide a direct means to observe convective velocity field in photosphere Should provide a direct means to observe convective velocity field in photosphere Cool stars bisectors resemble solar bisector, but with considerable variations Cool stars bisectors resemble solar bisector, but with considerable variations Gray & Nagel (1989) found bisectors reversed in hotter stars: a “granulation boundary” Gray & Nagel (1989) found bisectors reversed in hotter stars: a “granulation boundary” Two “different” types of convection?? Two “different” types of convection??

12 August 22, 2006IAU Symposium 239 Bisector curvature (asymmetry) - 2 On MS, reversed bisectors also found among A stars (T e <10500 K) On MS, reversed bisectors also found among A stars (T e <10500 K) Late B stars show no bisector curvature, and have  < 1 km/s Late B stars show no bisector curvature, and have  < 1 km/s Bisector curvature not studied for hotter stars, mainly because so few have v sin i < 5 km/s Bisector curvature not studied for hotter stars, mainly because so few have v sin i < 5 km/s

13 August 22, 2006IAU Symposium 239 Multi-parameter models of flow Modelling of cool stars by Dravins (1990) with four-component flow (2 hot upflows, 1 neutral, 1 cool downflow) reproduces line profiles reasonably and supports general picture of flow behaviour Modelling of cool stars by Dravins (1990) with four-component flow (2 hot upflows, 1 neutral, 1 cool downflow) reproduces line profiles reasonably and supports general picture of flow behaviour Frutiger et al (2000, 2005) have used multi- parameter models to derive temperature and velocity structure of simple geometrical flow models for Sun,  Cen A & B Frutiger et al (2000, 2005) have used multi- parameter models to derive temperature and velocity structure of simple geometrical flow models for Sun,  Cen A & B Useful for searches of parameter space Useful for searches of parameter space

14 August 22, 2006IAU Symposium 239 3D hydrodynamic models Physically realistic modelling requires 3D hydrodynamic models (e.g. Nordlund & Dravins) but such models are very costly Physically realistic modelling requires 3D hydrodynamic models (e.g. Nordlund & Dravins) but such models are very costly 3D models of low-metal stars with convection reveal that temperature stratification is changed significantly, perhaps also changing derived Li abundance (Asplund & Garcia Perez 2001) 3D models of low-metal stars with convection reveal that temperature stratification is changed significantly, perhaps also changing derived Li abundance (Asplund & Garcia Perez 2001)

15 August 22, 2006IAU Symposium 239 3D models - 2 Detailed model of Procyon allows comparison of micro- macro-turbulence fits to fits of 3D line profiles (Allende Prieto et al 2002) Detailed model of Procyon allows comparison of micro- macro-turbulence fits to fits of 3D line profiles (Allende Prieto et al 2002) Without free model parameters (except fundamental parameters of star), 3D model lines provide excellent fit to observations Without free model parameters (except fundamental parameters of star), 3D model lines provide excellent fit to observations

16 August 22, 2006IAU Symposium 239 3D models - 3 CO 5 BOLD code used to compute coarse model of entire M2 I star; find giant convection cells as suggested by images (Freytag et al 2002) CO 5 BOLD code used to compute coarse model of entire M2 I star; find giant convection cells as suggested by images (Freytag et al 2002) Same code computed convective models of A star, but found no reversed bisectors (Steffen et al 2005) Same code computed convective models of A star, but found no reversed bisectors (Steffen et al 2005) Limitation of 3D codes – if one disagrees with observation, testing changes is very costly Limitation of 3D codes – if one disagrees with observation, testing changes is very costly

17 August 22, 2006IAU Symposium 239 MLT and other convection models MLT, FST and non-local convection models provide alternative description MLT, FST and non-local convection models provide alternative description Comparisons of predictions of such models with Balmer lines, uvby colours of star (Smalley & Kupka 1997; Gardiner et al 1999) show that observational tests of models are possible Comparisons of predictions of such models with Balmer lines, uvby colours of star (Smalley & Kupka 1997; Gardiner et al 1999) show that observational tests of models are possible Kupka & Montgomery (2002) seem to predict correct sense of A star bisectors from non-local convection model Kupka & Montgomery (2002) seem to predict correct sense of A star bisectors from non-local convection model

18 August 22, 2006IAU Symposium 239 Conclusions Stellar atmospheric velocity fields clearly detectable in spectrum: microturbulence, macroturbulence, bisector curvature, energy distribution,…. Stellar atmospheric velocity fields clearly detectable in spectrum: microturbulence, macroturbulence, bisector curvature, energy distribution,…. Behaviour over HR diagram quite varied; largest velocities in supergiants Behaviour over HR diagram quite varied; largest velocities in supergiants Modelling making progress at connecting convection theory with observations Modelling making progress at connecting convection theory with observations


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